RFR(S): Refactor parallel ClassLoaderData iterator

[Zhengyu Gu]

Using additonal atomic variable and atomic operations to have all 
workers to walk CLD once, does not seem very profitable. ClassLoaderData 
itself supports parallel walking.

Also, cleanup a unnecessary ClassLoaderDataGraph::clear_claimed_marks()

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/par_cld/webrev.00/index.html


Test:

   fastdebug and release: hotspot_gc_shenandoah.


Thanks,

-Zhengyu